A particular carrier wave of the differentially biphase-coded type is characterized in that its phase is reversed at the beginning of every cycle in which the transmitted signal has the logical value "1" whereas the phase remains unchanged when that logical value is "0." With a slightly different code, described in commonly owned U.S. patent application Ser. No. 970,147, now U.S. Pat. No. 4,232,387, filed Dec. 18, 1978 by Ezio Cottatellucci, such a 180.degree. phase shift occurs only upon a change of the basic signal from "0" to "1" or vice versa.
The decoding of such a biphase-coded carrier wave can be performed, as disclosed in the above-identified Cottatellucci patent, by algebraically combining the squared or clipped wave with a replica thereof shifted by half a cycle to produce a ternary wave and sampling the latter midway in every other half-cycle of the original wave. The sampling operation can be timed with the aid of correlation pulses or spikes derived by differentiation from the incoming carrier wave, these spikes ocurring invariably in the middle of each cycle while also appearing at irregular intervals--depending upon the logical values of the message bits--at the beginning of a new cycle. The correct sampling times are determined from the recurring pattern of the spikes.
Two modes of distinguishing between the "right" and the "wrong" half-cycles for sampling purposes have been particularly disclosed in the Cottatellucci patent. According to the first mode, an error signal is generated when the squared carrier wave and its replica have the same magnitude at the instant of sampling. This technique is applicable only to the specific biphase code considered in that application and would have to be modified for a different code. The second mode, more universally applicable, involves the use of marker pulses, obtained from the correlation pulses through a phase shift by a fraction of a half-cycle, and the blocking of these marker pulses during alternate half-cycles under the control of a locally generated square wave of the same frequency as the carrier wave, the unblocked marker pulses giving rise to masking pulses of a half-cycle's duration which control a gate in an output branch of the sampling-pulse generator so as to blank all properly positioned sampling pulses; an improperly positioned sampling pulse, passed by the gate, again results in an error signal. With either mode, as a precaution against accidental phase shifts due to noise, the error signals are accumulated to switch the sampling times by one half-cycle only in the event of a predetermined error rate, i.e. upon surpassing a given threshold. The disclosure of that commonly owned application is hereby incorporated by reference into the present application.
Another way of decoding a biphase-coded carrier wave, especially one of the type first described hereinabove, utilizes a technique in which the clipped wave and its replica are phase shifted by a full cycle. In that instance the resulting composite wave is essentially a duplicate of the basic signal and may therefore be correctly sampled in either half of any cycle, except for the fact that distortion experienced by the wave during transmission may make sampling in one half-cycle less error-prone and therefore more desirable than in the other half-cycle.